The present invention relates to an optical amplifying repeater apparatus implemented by using as a repeater an optical amplifier which is constituted by employing an optical fiber doped with laser activating substance(s) such as rare-earth element(s), transition metal(s) or the like and also relates to an optical amplifying/repeating transmission system in which the optical amplifying repeater apparatuses are made use of.
When compared with the conventional optical repeater having 3R (Reshaping, Retiming, Regenerating) functions known heretofore, the optical fiber amplifier has desirable features such as independency on the transmission rate, susceptibility to simplified implementation of the repeater, possibility of implementation with large capacity owing to the wavelength multiplexing capability and others. Thus, the optical fiber amplif ier is expected to promise a key component which is capable of enhancing flexibility of the optical communication system. In particular, in an optical network in which the wavelength multiplexing technique is adopted, it is possible to achieve remarkable economization.
As the conventional or prior art optical amplifying repeater apparatus of the type mentioned above, there can be mentioned, for example, the one disclosed in xe2x80x9cOPTICAL AMPLIFIERS AND THEIR APPLICATIONxe2x80x9d, PP. 280-283, 1998. FIG. 12 is a block diagram showing the prior art optical amplifying repeater apparatus disclosed in the above-mentioned publication.
In FIG. 12, reference numerals 1; 4 denote optical amplifiers each of fixed gain type for amplifying en bloc light signals of wavelengths xcex1 to xcexn, numeral 3 denotes an adjustable optical attenuator, numeral 77 denotes an optical branching device for extracting a part of output power, numeral 6 denotes an optical attenuator control circuit for controlling the adjustable optical attenuator, numerals 501; 506 denote optical amplifiers, respectively, each implemented by making use of an erbium-doped fiber or the like, numerals 502 and 507 denote pumping light sources, respectively, numerals 503, 504, 508 and 509 denote optical branching devices for extracting parts of power of the light signals inputted thereto, respectively, and reference numerals 505 and 510 denote pumping light source control circuits for controlling the pumping light sources, respectively.
Next, description will be made of operation of the optical amplifying repeater apparatus. The wavelength-multiplexed light signals xcex1 to xcexn as inputted are first amplified by the fixed-gain optical amplifier 1 with a predetermined gain G0 and subsequently undergo attenuation with a predetermined attenuation factor through the adjustable optical attenuator 3. The wavelength-multiplexed light signals outputted from the adjustable optical attenuator 3 are again amplified by the fixed-gain optical amplifier 4 with a predetermined gain G1 to be ultimately outputted by way of the optical branching device 77. In that case, a part of the output signal is extracted through the optical branching device 77 and detected by the optical attenuator control circuit 6, which circuit is so designed as to control the factor of attenuation effectuated by the adjustable optical attenuator 3 so that the part of the output light signal extracted through the optical branching device 77 assumes a predetermined value. In this manner, the overall or total output power of the optical amplifying repeater apparatus is maintained at a constant value. In the case where the number of the wavelengths is constant, the output powers of the respective wavelengths can be maintained constant on a wavelength-by-wavelength basis, rendering it possible to realize ideal operation.
At this juncture, operation of the fixed-gain optical amplifier 1 will be described in detail. Input/output powers to/from the fixed-gain optical amplifier 1 are monitored through the optical branching devices 503 and 504, respectively, wherein the pumping light source control circuit 505 controls the pumping light source 102 such that the ratio between the input and output powers of the fixed-gain optical amplifier can be maintained to be constant. In this way, the gain of the fixed-gain optical amplifier 1 is held constant. Similar operation is performed for the fixed-gain optical amplifier 4 as well.
Furthermore, FIG. 13 shows in a block diagram another prior art optical amplifying repeater apparatus which is disclosed, for example, in xe2x80x9cOPTICAL AMPLIFIERS AND THEIR APPLICATIONSxe2x80x9d, MD1, 1998. This optical amplifying repeater apparatus is so arranged as to perform not only amplification of the light signals of wavelengths xcex1 to xcexn but also gain control for the optical repeater on the basis of monitoring information carried by a monitoring light signal xcexs sent from terminal equipment. In FIG. 13, reference numeral 11 denotes an optical branching device for separating the monitoring light signal xcexs from the light signals of wavelengths xcex1 to xcexn, and reference numeral 17 denotes a monitoring light receiver.
Next, description will turn to operation of the optical amplifying repeater apparatus described above. The wavelength-multiplexed light signals xcex1 to xcexn as inputted are first amplified by the fixed-gain optical amplifier 1 with a predetermined gain G0 to subsequently undergo attenuation with a predetermined attenuation factor through the adjustable optical attenuator 3. The wavelength-multiplexed light signals outputted from the adjustable optical attenuator 3 are again amplified by the fixed-gain optical amplifier 4 with a predetermined gain G1 to be outputted via the optical branching device 77. A part of the output signal is extracted through the optical branching device 77 and detected by the optical attenuator control circuit 6, which circuit is also so designed as to control the attenuation effectuated by the adjustable optical attenuator 3 so that the part of the output signal extracted through the optical coupling device 77 assumes a predetermined value. In this manner, the overall total output power of the optical amplifying repeater apparatus is maintained at a constant value or level. So long as the number of the wavelengths is constant, the output powers of the respective wavelengths can be maintained constant on a wavelength-by-wavelength basis, whereby ideal operation can be ensured. The information about the number of wavelengths is contained in the monitoring information carried by the monitoring light signal xcexs sent out from the terminal equipment and thus inputted to the optical attenuator control circuit 6 after reception by the monitoring light receiver 17.
In the optical amplifying repeater apparatus of the structures described above, a part of the total output power is extracted by the optical branching device 77 for the purpose of controlling the adjustable optical attenuator 3 so that the output power can be maintained to be constant. As a result of this, very troublesome procedure is required for coping with increase or decrease of the number of wavelengths. More specifically, because the total output power of the optical amplifying repeater apparatus depends on the number of wavelengths, there arises necessity of messaging in advance to the optical attenuator control circuit 6 the value which the light power extracted through the optical branching device 77 is to assume, when the number of wavelengths is changed. Consequently, in the case where one of the wavelength-multiplexed light signals of wavelengths xcex1 to xcexn is not transmitted due to some failure in sender equipment, by way of example, the messaging procedure such as mentioned above will not be in time for coping with the change of the number of wavelengths, thus bringing about corresponding changes in the powers of the other wavelengths, which of course will exert adverse influence to the quality of communication.
An object of the present invention is to solve the problem such as mentioned above and to provide an optical amplifying repeater apparatus which can positively protect the quality of communication from being degraded even when the powers of the other wavelengths change and further provides an optical amplifying/repeating transmission system which can maintain the system gain to be constant independently of the light signal power as inputted or the number of wavelengths thereof.
An optical amplifying repeater apparatus which the present invention concerns includes an input port to which a plurality of light signals multiplexed in wavelength are inputted, a first fixed-gain optical amplifier for amplifying the wavelength-multiplexed light signals inputted through the input port with a predetermined gain, an adjustable optical attenuator for attenuating the wavelength-multiplexed light signals by a predetermined factor after amplification through the first fixed-gain optical amplifier, an optical demultiplexer for demultiplexing the wavelength-multiplexed light signals outputted from the adjustable optical attenuator into a plurality of individual light signals of discrete wavelengths, respectively, fixed-gain optical amplifiers #1; #n for amplifying the plurality of individual wavelength-demultiplexed light signals outputted from the optical demultiplexer with a predetermined gain, a monitoring light branching device connected to one of outputs of the second fixed-gain optical amplifiers #1; #n for extracting a part of a specific monitoring light signal, an optical multiplexer for multiplexing the individual light signals outputted from the second fixed-gain optical amplifiers #1; #n, respectively, for thereby outputting multiplexed light signals, and an adjustable attenuator control circuit for detecting output power of the monitoring light branching device to thereby control the attenuation factor of the optical attenuator so that the detected output power assumes a constant value.
Further, an optical amplifying repeater apparatus which the present invention concerns includes an input port to which a plurality of light signals multiplexed in wavelength are inputted, a first fixed-gain optical amplifier for amplifying the wavelength-multiplexed light signals inputted through the input port with a predetermined gain, an optical demultiplexer connected to output of the first fixed-gain optical amplifier for demultiplexing the wavelength-multiplexed light signals into a plurality of individual light signals of respective wavelengths, adjustable optical attenuators #1; #n for attenuating the outputs of the optical demultiplexers, respectively, by a predetermined factor, fixed-gain optical amplifiers #1; #n for amplifying the plurality of light signals outputted from the adjustable optical attenuators with a predetermined gain, monitoring light branching devices connected to outputs of the fixed-gain optical amplifiers #1; #n for extracting parts of specific monitoring light signals #1; #n, respectively, an optical multiplexer for multiplexing the outputs of the fixed-gain optical amplifiers #1; #n to thereby output multiplexed light signals, and adjustable attenuator control circuits #1; #n for detecting output powers of the monitoring light branching devices to thereby control the attenuation factor of the optical attenuators so that each of the detected output powers assumes a constant value.
In the optical amplifying repeater apparatus described above, the first fixed-gain optical amplifier includes an optical fiber serving as a transmission line, a pumping light source for iinducing stimulated Raman amplifying action internally of the optical fiber in wavelength bands of the plurality of wavelength-multiplexed light signals, and an optical coupling device for coupling the output of the pumping light source with the plurality of wavelength-multiplexed light signals.
Further, in the optical amplifying repeater apparatus described above, the first fixed-gain optical amplifier includes an optical fiber doped with a rare-earth element or alternatively a transition metal, a pumping light source for inducing amplifying action under stimulated emission internally of the optical fiber in wavelength bands of the plurality of wavelength-multiplexed light signals, and an optical coupling device for coupling the output of the pumping light source with the plurality of wavelength-multiplexed light signals, wherein the optical fiber doped with the rare-earth element or alternatively the transition metal is operative in an unsaturated region.
Further, in the optical amplifying repeater apparatuses described above, the fixed-gain optical amplifier #1; #n includes an optical amplifying unit composed of an optical fiber doped with a rare-earth element or alternatively a transition metal and a pumping light source for stimulating the rare-earth element or alternatively the transition metal to thereby bring about stimulated emission, a compensating light source for generating compensating light having a wavelength within an amplified wavelength band of the optical amplifying unit, an compensating light coupling device for coupling the compensating light with the plurality of wavelength-multiplexed light signals, a compensating light branching device for separating mutually the compensating light and the plural wavelength-multiplexed light signals contained in the output of the optical amplifying unit, and a compensating light control circuit for controlling output power of the compensating light source such that ratio between power of the compensating light outputted from the compensating light branching device and power of the compensating light outputted from the compensating light source assumes a predetermined standard value.
Furthermore, in the optical amplifying repeater apparatuses described above, the fixed-gain optical amplifier #1; #n includes an optical amplifying unit composed of an optical fiber doped with a rare-earth element or alternatively a transition metal and a pumping light source for stimulating the rare-earth element or alternatively the transition metal to thereby bring about stimulated emission, and a compensating light control circuit for controlling output power of the compensating light source such that power of spontaneously emitted light outputted from the optical amplifying unit assumes a predetermined standard value.
Furthermore, in the optical amplifying repeater apparatuses described above, the fixed-gain optical amplifier#1; #n includes an adjustable optical attenuator #1xe2x80x2; #nxe2x80x2 inserted at an input or output side, whereby a means for changing the attenuation factor of the adjustable optical attenuator #1xe2x80x2; #nxe2x80x2 in dependence on ambient temperature is implemented.
Additionally, an optical amplifying/repeating transmission system which the present invention concerns includes a plurality of transmitters for sending out light signals of wavelengths differing one another and carrying information, a plurality of receivers for receiving the plurality of light signals of mutually different wavelengths, and a plurality of optical repeaters installed between the transmitters and the receivers for amplifying the plurality of light signals, and optical fibers interconnecting the transmitters and the optical repeater, the plurality of optical repeaters, and the optical repeater and the receivers, respectively, wherein the optical amplifying/repeating transmission system further includes a control signal superposing means for superposing a control signal of a specific frequency onto one of the plural light signals, a control signal level detecting means connected to the output of the optical repeater for extracting a part of output power of the optical repeater to thereby detect power of the control signal, and an optical repeater gain control means for controlling gain of the optical repeater so that level of the control signal detected by the control signal level detecting means remains constant.
Moreover, an optical amplifying/repeating transmission systemwhich the present invention concerns includes a plurality of transmitters for sending out light signals of wavelengths differing one another and carrying information, a plurality of receivers for receiving the plurality of light signals of mutually different wavelengths, and a plurality of optical repeaters installed between the transmitters and the receivers for amplifying the plurality of light signals, and optical fibers interconnecting the transmitters and the optical repeater, the plurality of optical repeaters, and the optical repeater and the receivers, respectively, wherein the optical amplifying/repeating transmission system further includes a control signal generating means for modulating one of the plural light signals with a control signal of a specific frequency, a control signal level detecting means connected to the output of the optical repeater for extracting a part of output power of the optical repeater to thereby detect power of the control signal, and an optical repeater gain control means for controlling gain of the optical repeater so that level of the control signal detected by the control signal level detecting means remains constant.
Besides, an optical amplifying/repeating transmission system which the present invention concerns includes a plurality of transmitters for sending out light signals of wavelengths differing one another and carrying information, a plurality of receivers for receiving the plurality of light signals of mutually different wavelengths, and a plurality of optical repeaters installed between the transmitters and the receivers for amplifying the plurality of light signals, and optical fibers interconnecting the transmitters and the optical repeater, the plurality of optical repeaters, and the optical repeater and the receivers, respectively, wherein the optical amplifying/repeating transmission system includes a control signal superposing means for superposing a control signal of a specific frequency onto one of the plural light signals, a control signal level detecting means connected to the output of the optical repeater for extracting a part of output power of the optical repeater to thereby detect power of the control signal, and an optical repeater gain control means for controlling gain of the optical repeater so that level of the control signal detected by the control signal level detecting means remains constant, a monitor signal transmitting means for supplying transmission level of the control signal to the optical repeater, and a monitor/control means for receiving a monitoring signal supplied from the monitor signal transmitting means to thereby enable the optical repeater gain control means to operate when the level of the control signal is normal while disabling operation of the optical repeater gain control means with the gain thereof being held when the level of the control signal is abnormal.
Besides, an optical amplifying/repeating transmission system according to the present invention includes a plurality of transmitters for sending out light signals of wavelengths differing one another and carrying information, a plurality of receivers for receiving the plurality of light signals of mutually different wavelengths, and a plurality of optical repeaters installed between the transmitters and the receivers for amplifying the plurality of light signals, and optical fibers interconnecting the transmitters and the optical repeater, the plurality of optical repeaters, and the optical repeater and the receivers, respectively, wherein the optical amplifying/repeating transmission system further includes a control signal generating means for modulating one of the plural light signals with a control signal of a specific frequency, a control signal level detecting means connected to the output of the optical repeater for extracting a part of output power of the optical repeater to thereby detect power of the control signal, and an optical repeater gain control means for controlling gain of the optical repeater so that level of the control signal detected by the control signal level detecting means remains constant, a monitor signal transmitting means for supplying transmission level of the control signal to the optical repeater, and a monitor/control means for receiving a monitoring signal supplied from the monitor signal transmitting means to thereby enable the optical repeater gain control means to operate when the level of the control signal is normal while disabling operation of the optical repeater gain control means with the gain thereof being held when the level of the control signal is abnormal.
In the optical communication system described above, the optical repeater gain control means is composed of an adjustable optical attenuator and a means for controlling the adjustable optical attenuator.
Further, in the optical communication system described above, the control signal has a frequency higher than 100 kHz inclusive thereof.
Furthermore, in the optical communication system described above, the monitoring signal has a wavelength shorter than those sent out from the plurality of transmitters.
Moreover, an optical amplifying repeater apparatus for amplifying a plurality of wavelength-multiplexed light signals according to the present invention includes a first fixed-gain optical amplifier for amplifying a plurality of wavelength-multiplexed light signals inputted thereto with a predetermined gain, a control light signal branching device for extracting a part of power of a control wavelength contained in the output of the first fixed-gain optical amplifier, an adjustable optical attenuator for attenuating the output of the first fixed-gain optical amplifier by a predetermined factor, an adjustable attenuator control circuit for detecting output power of the control light signal branching device to thereby control the attenuation factor of the adjustable optical attenuator so that the detected output power remains constant, and a second fixed-gain optical amplifier connected to output of the adjustable optical attenuator for amplifying the plural wavelength-multiplexed light signal inputted to the second fixed-gain optical amplifier with a predetermined gain.
In the optical amplifying repeater apparatus described above further includes an output breaking circuit for stopping amplifying function of the second fixed-gain optical amplifier upon detection of disappearance of output of the control light signal branching device.